Heat exchanger beds composed of thermomagnetic material

ABSTRACT

Provided is a packed heat exchanger bed composed of thermomagnetic material particles having a mean particle diameter of 50 μm to 1 mm. The packed bed has porosity of 30-45%. A thermomagnetic material is a metal containing material such as (A y B 1-y ) 2+δ C w D x E z , La(Fe x Al 1-x ) 13 H y  or La(Fe x Si 1-x ) 13 H y , La(Fe x Al y Co z ) 13  or La(FeSi y Co z ) 13 , LaMn x Fe 2-x Ge, Heusler alloys, Gd 5 (Si x Ge 1-x ) 4 , Fe 2 P-based compounds, manganites of the perovskite type, Tb 5 (Si 4-x Ge x ), XTiGe, Mn 2-x Z x Sb, or Mn 2 Z x Sb 1-x , wherein A- E, P, and Z represent various metal atoms.

This is a continuation of U.S. application Ser. No. 12/850,891, filedAug. 5, 2010, which claimed priority to European patent application no.09167550.4, filed Aug. 10, 2009, of which all of the disclosures areincorporated herein by reference in their entireties.

DESCRIPTION

The invention relates to packed heat exchanger beds composed ofthermomagnetic material particles or composed of a thermomagneticmaterial monolith, to processes for production thereof and to the usethereof in refrigerators, air conditioning units, heat pumps or in powergeneration by direct conversion of heat.

BACKGROUND OF THE INVENTION

Thermomagnetic materials, also referred to as magnetocaloric materials,can be used for cooling, for example in refrigerators or airconditioning units, in heat pumps or for direct generation of power fromheat without intermediate connection of a conversion to mechanicalenergy.

Such materials are known in principle and are described, for example, inWO 2004/068512. Magnetic cooling techniques are based on themagnetocaloric effect (MCE) and may constitute an alternative to theknown vapor circulation cooling methods. In a material which exhibits amagnetocaloric effect, the alignment of randomly aligned magneticmoments by an external magnetic field leads to heating of the material.This heat can be removed from the MCE material to the surroundingatmosphere by a heat transfer. When the magnetic field is then switchedoff or removed, the magnetic moments revert back to a randomarrangement, which leads to cooling of the material below ambienttemperature. This effect can be exploited for cooling purposes; see alsoNature, Vol. 415, Jan. 10, 2002, pages 150 to 152. Typically, a heattransfer medium such as water is used for heat removal from themagnetocaloric material.

The materials used in thermomagnetic generators are likewise based onthe magnetocaloric effect. In a material which exhibits a magnetocaloriceffect, the alignment of randomly aligned magnetic moments by anexternal magnetic field leads to heating of the material. This heat canbe released by the MCE material into the surrounding atmosphere by aheat transfer. When the magnetic field is then switched off or removed,the magnetic moments revert back to a random alignment, which leads tocooling of the material below ambient temperature. This effect can beexploited firstly for cooling purposes, and secondly for conversion ofheat to electrical energy.

The magnetocaloric generation of electrical energy is associated withmagnetic heating and cooling. At the time of first conception, theprocess for energy generation was described as pyromagnetic energygeneration. Compared to devices of the Peltier or Seebeck type, thesemagnetocaloric devices can have a significantly higher energyefficiency.

The research into this physical phenomenon began in the late 19^(th)century, when two scientists, Tesla and Edison, filed a patent onpyromagnetic generators.

For the thermomagnetic or magnetocaloric applications, the materialshould permit efficient heat exchange in order to be able to achievehigh efficiencies. Both in the course of cooling and in the course ofpower generation, the thermomagnetic material is used in a heatexchanger.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide heat exchanger bedscomposed of thermomagnetic shaped bodies which are suitable for use inheat exchangers, especially for cooling purposes or for powergeneration. These shaped bodies should allow high heat transfer, have alow flow resistance for heat exchange media and possess a highmagnetocaloric density.

The object is achieved in accordance with the invention by a packed heatexchanger bed composed of thermomagnetic material particles which have amean diameter in the range from 50 μm to 1 mm and give rise to aporosity in the packed bed in the range from 30 to 45%.

The porosity is defined as the proportion by volume of empty space(interstices) in the heat exchanger bed.

The heat exchanger bed can be produced by a process in which a powder ofthe thermomagnetic material is subjected to shaping to form thethermomagnetic material particles and the material particles aresubsequently packed to form the heat exchanger bed.

The object is additionally achieved by a heat exchanger bed composed ofa thermomagnetic material monolith which has continuous channels with across-sectional area of the individual channels in the range from 0.001to 0.2 mm² and a wall thickness of 50 to 300 μm, a porosity in the rangefrom 10 to 60% and a ratio of surface to volume in the range from 3000to 50 000 m²/m³.

Alternatively, the thermomagnetic material monolith may comprise or beformed from a plurality of parallel sheets with a sheet thickness of 0.1to 2 mm, preferably 0.5 to 1 mm, and a plate separation (interstice) of0.05 to 1 mm, preferably 0.05 to 0.2 mm. The number of sheets may, forexample, be 5 to 100, preferably 10 to 50.

The heat exchanger bed is produced, for example, by extrusion, injectionmolding or molding of the thermomagnetic material to form the monolith.

The object is additionally achieved by the use of a heat exchanger bedas defined above in refrigerators, air conditioning units, heat pumps orin power generation by direct conversion of heat.

It has been found in accordance with the invention that a packed heatexchanger bed composed of thermomagnetic material particles is a highlyefficient material geometry which allows optimal operation of the heatexchanger bed when the thermomagnetic material particles have a meandiameter in the range from 50 μm to 1 mm and there is a porosity in thepacked bed in the range from 30 to 45%. The individual materialparticles may have any desired form. The material particles arepreferably in spherical form, pellet form, sheet form or cylinder form.The material particles are more preferably in spherical form. Thediameter of the material particles, especially of the spheres, is 50 μmto 1 mm, more preferably 200 to 400 μm. The material particles,especially spheres, may have a size distribution. The size distributionis preferably narrow, such that predominantly spheres of one size arepresent. The diameter preferably differs from the mean diameter by notmore than 20%, more preferably by not more than 10%, especially by notmore than 5%.

In the packed bed, this results in a porosity in the range from 30 to45%, more preferably from 36 to 40%.

Material particles, especially spheres with the above dimensions, as apacked heat exchanger bed, give high heat transfer coefficients betweensolid and fluid (heat exchanger fluid), the pressure drop being low.This allows an improved coefficient of performance (COP) of the heatexchanger bed. The high heat transfer coefficient allows the packed bedsto be operated at higher frequencies than customary, and hence allowsgreater energy extraction.

It is possible for a single thermomagnetic material to be present in thepacked heat exchanger beds, but it is also possible that a series ofdifferent magnetocaloric materials with different Curie temperatures arecombined. This allows a large temperature change overall to be achievedin a single heat exchanger bed. Preference is given in accordance withthe invention to combining thermomagnetic materials whose maximumdifference in Curie temperature is 1 to 10° C., more preferably 2 to 6°C.

For the particular operating conditions, the performance of the packedheat exchanger bed can be optimized by using material particles,especially spheres, of different diameter. A lower diameter, especiallysphere diameter, leads to a higher coefficient of heat transfer andhence allows better heat exchange. This, however, is associated with ahigher pressure drop through the heat exchanger bed. Conversely, the useof larger material particles, especially spheres, leads to slower heattransfer, but to lower pressure drops.

The packed heat exchanger bed composed of the thermomagnetic materialparticles can be produced in any suitable manner. The thermomagneticmaterial particles are first produced, for example by shaping a powderof the thermoelectric material to form the thermomagnetic materialparticles. Subsequently, the material particles are packed to form theheat exchanger bed. This can be done by pouring the material particlesinto a suitable vessel, in which case the settling of the bed can beimproved by shaking. Floating in a fluid with subsequent settling of thematerial particles is also possible. It is additionally possible tosettle the individual material particles in a controlled manner to forma homogeneous structure. In this case, it is possible, for example, toachieve a tight cubic packing of spheres.

The movement resistance of the packed heat exchanger bed can be achievedby any suitable measures. For example, the vessel in which the packedheat exchanger bed is present can be closed on all sides. This can bedone, for example, using a mesh cage. In addition, it is possible tojoin the individual material particles to one another, for example bysurface melting of the material particles in the packed bed or bysintering the material particles to one another in the packed bed. Thesurface melting or sintering should be effected such that theinterstices between the material particles are very substantiallypreserved.

The formation of the packed heat exchanger bed by thermomagneticmaterial particles in sheet, cylinder, pellet or sphere form or similarform is advantageous, since a large ratio of surface to mass is achievedtherewith. This achieves an improved heat transfer rate coupled withrelatively low pressure drop.

A second advantageous embodiment of the heat exchanger bed is athermomagnetic material monolith which has continuous channels. Themonolith can be considered as a block of thermomagnetic material, inwhich case two opposite end sides of the block have entry and exitorifices for a fluid which are connected by channels which run throughthe entire monolith. Corresponding monoliths can be derived, forexample, from a tube bundle in which the individual tubes ofthermomagnetic material are joined to one another. The channels arepreferably parallel to one another and generally run through themonoliths in a straight line. When particular use requirements are made,it is also possible to provide a curved profile of the channels.Corresponding monolith forms are known, for example, from automotiveexhaust gas catalysts. The thermomagnetic material monoliths may thushave, for example, a cellular form, in which case the individual cellsmay have any desired geometry. For example, the channels may have ahexagonal cross section as in the case of a honeycomb, or a rectangularcross section. Star-shaped cross sections, round cross sections, ovalcross sections or other cross sections are also possible in accordancewith the invention, provided that the following conditions are observed:

-   -   cross-sectional area of the individual channels in the range        from 0.001 to 0.2 mm², more preferably 0.01 to 0.03 mm²,        especially 0.015 to 0.025 mm²    -   wall thickness of 50 to 300 μm, more preferably 50-150 μm,        especially 85 to 115 μm    -   porosity in the range from 10 to 60%, more preferably 15 to 35%,        especially 20 to 30%    -   ratio of surface to volume in the range from 3000 to 50 000        m²/m³, more preferably 5000 to 15 000 m²/m³.

The individual channels may have, for example, with a rectangular crosssection, cross-sectional dimensions of 50 μm×25 μm to 600 μm×300 μm,especially about 200 μm×100 μm. The wall thickness may especiallypreferably be about 100 μm. The porosity may more preferably be about25%. The porosity is thus typically significantly lower than theporosity of a packed sphere bed. This allows more magnetocaloricmaterial to be introduced into a given volume of the magnetic field.This leads to a greater thermal effect with equal expenditure to providethe magnetic field.

The shaped bodies have continuous channels. This allows a liquid heatcarrier medium to flow through, such as water, water/alcohol mixtures,water/salt mixtures or gases such as air or noble gases. Preference isgiven to using water or water/alcohol mixtures, in which case thealcohol may be a mono- or polyhydric alcohol. For example, the alcoholsmay be glycols.

The monoliths may be formed, for example, from layers of magnetocaloricmaterial with thin parallel channels in the layers.

The very large ratio of surface to volume allows excellent heattransfer, coupled with a very low pressure drop. The pressure drop is,for instance, one order of magnitude lower than for a packed bed ofspheres which has the identical heat transfer coefficient. The monolithform thus allows the coefficient of performance (COP), for example of amagnetocaloric cooling device, to be improved considerably once again.

The thermomagnetic material itself may be selected from any suitablethermomagnetic materials. Suitable materials are described in amultitude of documents, for example in WO 2004/068512.

Preferred thermomagnetic materials are selected from

-   (1) compounds of the general formula (I)

(A_(y)B_(1-y))_(2+δ)C_(w)D_(x)E_(z)   (I)

-   -   where    -   A is Mn or Co,    -   B is Fe, Cr or Ni,    -   C, D and E at least two of C, D and E are different, have a        non-vanishing concentration and are selected from P, B, Se, Ge,        Ga, Si, Sn, N, As and Sb, where at least one of C, D and E is Ge        or Si,    -   δ is a number in the range from −0.1 to 0.1,    -   w, x, y, z are numbers in the range from 0 to 1, where w+x+z=1;

-   (2) La- and Fe-based compounds of the general formulae (II)    and/or (III) and/or (IV)

La(Fe_(x)Al_(1-x))₁₃H_(y) or La(Fe_(x)Si_(1-x))₁₃H_(y)   (II)

-   -   where    -   x is a number from 0.7 to 0.95,    -   y is a number from 0 to 3, preferably from 0 to 2;

La(Fe_(x)Al_(y)Co_(z))₁₃ or La(Fe_(x)Si_(y)Co_(z))₁₃   (III)

-   -   where    -   x is a number from 0.7 to 0.95,    -   y is a number from 0.05 to 1−x,    -   z is a number from 0.005 to 0.5;

LaMn_(x)Fe_(2-x)Ge   (IV)

-   -   where    -   x is a number from 1.7 to 1.95 and

-   (3) Heusler alloys of the MnTP type where T is a transition metal    and P is a p-doping metal having an electron count per atom e/a in    the range from 7 to 8.5,

-   (4) Gd- and Si-based compounds of the general formula (V)

Gd₅(Si_(x)Ge_(1-x))₄   (V)

-   -   where x is a number from 0.2 to 1,

-   (5) Fe₂P-based compounds,

-   (6) manganites of the perovskite type,

-   (7) compounds which comprise rare earth elements and are of the    general formulae (VI) and (VII)

Tb₅(Si_(4-x)Ge_(x))   (VI)

-   -   where x=0, 1, 2, 3, 4,

XTiGe   (VII)

-   -   where X=Dy, Ho, Tm,

-   (8) Mn- and Sb- or As-based compounds of the general formulae (VIII)    and (IX)

Mn_(2-x)Z_(x)Sb   (VIII)

Mn₂Z_(x)Sb_(1-x)   (IX)

-   -   where    -   Z is Cr, Cu, Zn, Co, V, As, Ge,    -   x is from 0.01 to 0.5,    -   where Sb may be replaced by As when Z is not As.

It has been found in accordance with the invention that theaforementioned thermomagnetic materials can be used advantageously inheat exchangers, magnetic cooling, heat pumps or thermomagneticgenerators or regenerators when they have an inventive structure.

Particular preference is given in accordance with the invention to themetal-based materials selected from compounds (1), (2) and (3), and also(5).

Materials particularly suitable in accordance with the invention aredescribed, for example, in WO 2004/068512, Rare Metals, Vol. 25, 2006,pages 544 to 549, J. Appl. Phys. 99, 08Q107 (2006), Nature, Vol. 415,Jan. 10, 2002, pages 150 to 152 and Physica B 327 (2003), pages 431 to437.

In the aforementioned compounds of the general formula (I), C, D and Eare preferably identical or different and are selected from at least oneof P, Ge, Si, Sn and Ga.

The thermomagnetic material of the general formula (I) is preferablyselected from at least quaternary compounds which, as well as Mn, Fe, Pand optionally Sb, additionally comprise Ge or Si or As or Ge and Si, Geand As or Si and As, or Ge, Si and As.

Preferably at least 90% by weight, more preferably at least 95% byweight, of component A is Mn. Preferably at least 90% by weight, morepreferably at least 95% by weight, of B is Fe. Preferably at least 90%by weight, more preferably at least 95% by weight, of C is P. Preferablyat least 90% by weight, more preferably at least 95% by weight, of D isGe. Preferably at least 90% by weight, more preferably at least 95% byweight, of E is Si.

The material preferably has the general formula MnFe(P_(w)Ge_(x)Si_(z)).

x is preferably a number in the range from 0.3 to 0.7, w is less than orequal to 1−x and z corresponds to 1−x−w.

The material preferably has the crystalline hexagonal Fe₂P structure.Examples of suitable materials are MnFeP_(0.45 to 0.7),Ge_(0.55 to 0.30) and MnFeP_(0.5 to 0.70), (Si/Ge)_(0.5 to 0.30).

Suitable compounds are additionally M_(n1+x)Fe_(1-x)P_(1-y)Ge_(y) with xin the range from −0.3 to 0.5, y in the range from 0.1 to 0.6. Likewisesuitable are compounds of the general formulaMn_(1+x)Fe_(1-x)P_(1-y)Ge_(y-z)Sb_(z) with x in the range from −0.3 to0.5, y in the range from 0.1 to 0.6 and z less than y and less than 0.2.Also suitable are compounds of the formulaMn₁₊xFe_(1-x)P_(1-y)Ge_(y-z)Si_(z) with x in the range from 0.3 to 0.5,y in the range from 0.1 to 0.66, z less than or equal to y and less than0.6.

Also suitable are further Fe₂P-based compounds proceeding from Fe₂P andFeAs₂, optionally Mn and P. They correspond, for example, to the generalformulae MnFe_(1-x)Co_(x)Ge, where x=0.7−0.9, Mn_(5-x)Fe_(x)Si₃ wherex=0−5, Mn₅Ge_(3-x)Si_(x) where x=0.1−2, Mn₅Ge_(3-x)Sb_(x) where x=0−0.3,Mn_(2-x)Fe_(x)Ge₂ where x=0.1−0.2, Mn_(3-x)Co_(x)GaC where x=0−0.05.

Preferred La- and Fe-based compounds of the general formulae (II) and/or(III) and/or (IV) are La(Fe_(0.90)Si_(0.10))₁₃,La(Fe_(0.89)Si_(0.11))₁₃, La(Fe_(0.880)Si_(0.120))₁₃,La(Fe_(0.877)Si_(0.123))₁₃, LaFe_(11.8)Si_(1.2),La(Fe_(0.88)Si_(0.12))₁₃H_(0.5), La(Fe_(0.88)Si_(0.12))₁₃H_(1.0),LaFe_(11.7)Si_(1.3)H_(1.1), LaFe_(11.57)Si_(1.43)H_(1.3),La(Fe_(0.88)Si_(0.12))H_(1.5), La(Fe_(11.2)Co_(0.7)Si_(1.1),LaFe_(11.5)Al_(1.5)C_(0.1), LaFe_(11.5)Al_(1.5)C_(0.2),LaFe_(11.5)Al_(1.5)C_(0.4), LaFe_(11.5)Al_(1.5)Co_(0.5),La(Fe_(0.94)Co_(0.06))_(11.83)Al_(1.17),La(Fe_(0.92)Co_(0.08))_(11.83)Al_(1.17).

Suitable manganese-comprising compounds are MnFeGe,MnFe_(0.9)Co_(0.1)Ge, MnFe_(0.8)Co_(0.2)Ge, MnFe_(0.7)Co_(0.3)Ge,MnFe_(0.6)Co_(0.4)Ge, MnFe_(0.5)Co_(0.5)Ge, MnFe_(0.4)Co_(0.6)Ge,MnFe_(0.3)Co_(0.7)Ge, MnFe_(0.2)Co_(0.8)Ge, MnFe_(0.15)Co_(0.85)Ge,MnFe_(0.1)Co_(0.9)Ge, MnCoGe, Mn₅Ge_(2.5)Si_(0.5), Mn₅Ge₂Si,Mn₅Ge_(1.5)Si_(1.5), Mn₅GeSi₂, Mn₅Ge₃, Mn₅Ge_(2.9)Sb_(0.1),Mn₅Ge_(2.8)Sb_(0.2), Mn₅Ge_(2.7)Sb_(0.3), LaMn_(1.9)Fe_(0.1)Ge,LaMn_(1.85)Fe_(0.15)Ge, LaMn_(1.8)Fe_(0.2)Ge, (Fe_(0.9)Mn_(0.1))₃C,(Fe_(0.8)Mn_(0.2))₃C, (Fe_(0.7)Mn_(0.3))₃C, Mn₃GaC, MnAs, (Mn, Fe)As,Mn_(1+δ)As_(0.8)Sb_(0.2), MnAs_(0.75)Sb_(0.25),Mn_(1.1)As_(0.75)Sb_(0.25), Mn_(1.5)As_(0.75)Sb_(0.25).

Heusler alloys suitable in accordance with the invention are, forexample, Ni₂MnGa, Fe₂MnSi_(1-x)Ge_(x) with x=0−1 such asFe₂MnSi_(0.5)Ge_(0.5), Ni_(52.5)Mn_(22.4)Ga_(24.7),Ni_(50.9)Mn_(24.7)Ga_(24.4), Ni_(55.2)Mn_(18.6)Ga_(26.2),Ni_(51.6)Mn_(24.7)Ga_(23.8), Ni_(52.7)Mn_(23.5)Ga_(23.4), CoMnSb,CoNb_(0.2)Mn_(0.8)Sb, CoNb_(0.4)Mn_(0.6)SB, CoNb_(0.6)Mn_(0.4)Sb,Ni₅₀Mn₃₅Sn₁₅, Ni₅₀Mn₃₇Sn₁₃, MnFeP_(0.45)As_(0.55),MnFeP_(0.47)As_(0.53), Mn_(1.1)Fe_(0.9)P_(0.47)As_(0.53),MnFeP_(0.89-x)Si_(x)Ge_(0.11), X=0.22, X=0.26, X=0.30, X=0.33.

Additionally suitable are Fe₉₀Zr₁₀, Fe₈₂Mn₈Zn₁₀, Co₆₆Nb₉Cu₁Si₁₂B₁₂,Pd₄₀M_(22.5)Fe_(17.5)P₂₀, FeMoSiBCuNb, Gd₇₀Fe₃₀, GdNiAl, NdFe₁₂B₆GdMn₂.

Manganites of the perovskite type are, for example,La_(0.6)Ca_(0.4)MnO₃, La_(0.67)Ca_(0.33)MnO₃, La_(0.8)Ca_(0.2)MnO₃,La_(0.7)Ca_(0.3)MnO₃, La_(0.958)Li_(0.025)Ti_(0.1)Mn_(0.9)O₃,La0.65Ca0.35Ti_(0.1)Mn_(0.9)O₃, La_(0.799)Na_(0.199)MnO_(2.97),La_(0.88)Na_(0.099)Mn_(0.977)O₃, La_(0.877)K_(0.096)Mn_(0.974)O₃,La_(0.65)Sr_(0.35)Mn_(0.95)Cn_(0.05)O₃, La_(0.7)Nd_(0.1)Na_(0.2)MnO₃,La_(0.5)Ca_(0.3)Sr_(0.2)MnO₃.

Gd- and Si-based compounds of the general formula (V)

Gd₅(Si_(x)Ge_(1-x))₄

where x is a number from 0.2 to 1

are, for example, Gd₅(Si_(0.5)Ge_(0.5))₄, Gd₅(Si_(0.425)Ge0.575)₄,Gd₅(Si_(0.45)Ge_(0.55))₄, Gd₅(Si_(0.365)Ge_(0.635))₄,Gd₅(Si_(0.3)Ge_(0.7))₄, Gd₅(Si_(0.25)Ge_(0.75))₄.

Compounds comprising rare earth elements are Tb₅(Si_(4-x)Ge_(x)) withx=0, 1, 2, 3, 4 or XTiGe with X=Dy, Ho, Tm, for example Tb₅Si₄,Tb₅(Si₃Ge), Tb(Si₂Ge₂), Tb₅Ge₄, DyTiGe, HoTiGe, TmTiGe.

Mn- and Sb- or As-based compounds of the general formulae (VIII) and(IX) preferably have the definitions of z=0.05 to 0.3, Z=Cr, Cu, Ge, As,Co.

The thermomagnetic materials used in accordance with the invention canbe produced in any suitable manner.

The thermomagnetic materials are produced, for example, by solid phasereaction of the starting elements or starting alloys for the material ina ball mill, subsequent pressing, sintering and heat treatment underinert gas atmosphere and subsequent slow cooling to room temperature.Such a process is described, for example, in J. Appl. Phys. 99, 2006,08Q107.

Processing via melt spinning is also possible. This makes possible amore homogeneous element distribution which leads to an improvedmagnetocaloric effect; cf. Rare Metals, Vol. 25, October 2006, pages 544to 549. In the process described there, the starting elements are firstinduction-melted in an argon gas atmosphere and then sprayed in themolten state through a nozzle onto a rotating copper roller. Therefollows sintering at 1000° C. and slow cooling to room temperature.

In addition, reference may be made to WO 2004/068512 for the production.

The materials obtained by these processes frequently exhibit highthermal hysteresis. For example, in compounds of the Fe₂P typesubstituted by germanium or silicon, large values for thermal hysteresisare observed within a wide range of 10 K or more.

Preference is therefore given to a process for producing thethermomagnetic materials, comprising the following steps:

-   -   a) reacting chemical elements and/or alloys in a stoichiometry        which corresponds to the metal-based material in the solid        and/or liquid phase,    -   b) if appropriate converting the reaction product from stage a)        to a solid,    -   c) sintering and/or heat treating the solid from stage a) or b),    -   d) quenching the sintered and/or heat-treated solid from        stage c) at a cooling rate of at least 100 K/s.

The thermal hysteresis can be reduced significantly and a largemagnetocaloric effect can be achieved when the metal-based materials arenot cooled slowing to ambient temperature after the sintering and/orheat treatment, but rather are quenched at a high cooling rate. Thiscooling rate is at least 100 K/s. The cooling rate is preferably from100 to 10 000 K/s, more preferably from 200 to 1300 K/s. Especiallypreferred cooling rates are from 300 to 1000 K/s.

The quenching can be achieved by any suitable cooling processes, forexample by quenching the solid with water or aqueous liquids, forexample cooled water or ice/water mixtures. The solids can, for example,be allowed to fall into ice-cooled water. It is also possible to quenchthe solids with subcooled gases such as liquid nitrogen. Furtherprocesses for quenching are known to those skilled in the art. What isadvantageous here is controlled and rapid cooling.

The rest of the production of the thermomagnetic materials is lesscritical, provided that the last step comprises the quenching of thesintered and/or heat-treated solid at the inventive cooling rate. Theprocess may be applied to the production of any suitable thermomagneticmaterials for magnetic cooling, as described above.

In step (a) of the process, the elements and/or alloys which are presentin the later thermomagnetic material are converted in a stoichiometrywhich corresponds to the thermomagnetic material in the solid or liquidphase.

Preference is given to performing the reaction in stage a) by combinedheating of the elements and/or alloys in a closed vessel or in anextruder, or by solid phase reaction in a ball mill. Particularpreference is given to performing a solid phase reaction, which iseffected especially in a ball mill. Such a reaction is known inprinciple; cf. the documents cited above. Typically, powders of theindividual elements or powders of alloys of two or more of theindividual elements which are present in the later thermomagneticmaterial are mixed in pulverulent form in suitable proportions byweight. If necessary, the mixture can additionally be ground in order toobtain a microcrystalline powder mixture. This powder mixture ispreferably heated in a ball mill, which leads to further comminution andalso good mixing, and to a solid phase reaction in the powder mixture.Alternatively, the individual elements are mixed as a powder in theselected stoichiometry and then melted.

The combined heating in a closed vessel allows the fixing of volatileelements and control of the stoichiometry. Specifically in the case ofuse of phosphorus, this would evaporate easily in an open system.

The reaction is followed by sintering and/or heat treatment of thesolid, for which one or more intermediate steps can be provided. Forexample, the solid obtained in stage a) can be subjected to shapingbefore it is sintered and/or heat treated.

Alternatively, it is possible to send the solid obtained from the ballmill to a melt-spinning process. Melt-spinning processes are known perse and are described, for example, in Rare Metals, Vol. 25, October2006, pages 544 to 549, and also in WO 2004/068512.

In these processes, the composition obtained in stage a) is melted andsprayed onto a rotating cold metal roller. This spraying can be achievedby means of elevated pressure upstream of the spray nozzle or reducedpressure downstream of the spray nozzle. Typically, a rotating copperdrum or roller is used, which can additionally be cooled if appropriate.The copper drum preferably rotates at a surface speed of from 10 to 40m/s, especially from 20 to 30 m/s. On the copper drum, the liquidcomposition is cooled at a rate of preferably from 10² to 10⁷ K/s, morepreferably at a rate of at least 10⁴ K/s, especially with a rate of from0.5 to 2×10⁶ K/s.

The melt-spinning, like the reaction in stage a) too, can be performedunder reduced pressure or under an inert gas atmosphere.

The melt-spinning achieves a high processing rate, since the subsequentsintering and heat treatment can be shortened. Specifically on theindustrial scale, the production of the thermomagnetic materials thusbecomes significantly more economically viable. Spray-drying also leadsto a high processing rate. Particular preference is given to performingmelt spinning.

Alternatively, in stage b), spray cooling can be carried out, in which amelt of the composition from stage a) is sprayed into a spray tower. Thespray tower may, for example, additionally be cooled. In spray towers,cooling rates in the range from 10³ to 10⁵ K/s, especially about 10⁴K/s, are frequently achieved.

The sintering and/or heat treatment of the solid is effected in stage c)preferably first at a temperature in the range from 800 to 1400° C. forsintering and then at a temperature in the range from 500 to 750° C. forheat treatment. For example, the sintering can then be effected at atemperature in the range from 500 to 800° C. For shaped bodies/solids,the sintering is more preferably effected at a temperature in the rangefrom 1000 to 1300° C., especially from 1100 to 1300° C. The heattreatment can then be effected, for example, at from 600 to 700° C.

The sintering is performed preferably for a period of from 1 to 50hours, more preferably from 2 to 20 hours, especially from 5 to 15hours. The heat treatment is performed preferably for a period in therange from 10 to 100 hours, more preferably from 10 to 60 hours,especially from 30 to 50 hours. The exact periods can be adjusted to thepractical requirements according to the materials.

In the case of use of the melt-spinning process, the period forsintering or heat treatment can be shortened significantly, for exampleto periods of from 5 minutes to 5 hours, preferably from 10 minutes to 1hour. Compared to the otherwise customary values of 10 hours forsintering and 50 hours for heat treatment, this results in a major timeadvantage.

The sintering/heat treatment results in partial melting of the particleboundaries, such that the material is compacted further.

The melting and rapid cooling in stage b) thus allows the duration ofstage c) to be reduced considerably. This also allows continuousproduction of the thermomagnetic materials.

The pressing can be carried out, for example, as cold pressing or as hotpressing. The pressing may be followed by the sintering process alreadydescribed.

In the sintering process or sintered metal process, the powders of thethermomagnetic material are first converted to the desired shape of theshaped body, and then bonded to one another by sintering, which affordsthe desired shaped body. The sintering can likewise be carried out asdescribed above.

It is also possible in accordance with the invention to introduce thepowder of the thermomagnetic material into a polymeric binder, tosubject the resulting thermoplastic molding material to a shaping, toremove the binder and to sinter the resulting green body. It is alsopossible to coat the powder of the thermomagnetic material with apolymeric binder and to subject it to shaping by pressing, ifappropriate with heat treatment.

According to the invention, it is possible to use any suitable organicbinders which can be used as binders for thermomagnetic materials. Theseare especially oligomeric or polymeric systems, but it is also possibleto use low molecular weight organic compounds, for example sugars.

The thermomagnetic powder is mixed with one of the suitable organicbinders and filled into a mold. This can be done, for example, bycasting or injection molding or by extrusion. The polymer is thenremoved catalytically or thermally and sintered to such an extent that aporous body with monolith structure is formed.

Hot extrusion or metal injection molding (MIM) of the thermomagneticmaterial is also possible, as is construction from thin sheets which areobtainable by rolling processes. In the case of injection molding, thechannels in the monolith have a conical shape, in order to be able toremove the moldings from the mold. In the case of construction fromsheets, all channel walls can run in parallel.

The particular processes are controlled so as to result in heatexchanger beds which have a suitable combination of high heat transfer,low flow resistance and high magnetocaloric density. Preference is givento an optimal ratio of high magnetocaloric density and sufficientporosity, so as to ensure efficient heat removal and efficient heatexchange. In other words, the inventive shaped bodies exhibit a highratio of surface to volume. By virtue of the high surface area, it ispossible to transport large amounts of heat out of the material and totransfer them into a heat transfer medium. The structure should bemechanically stable in order to cope with the mechanical stresses by afluid cooling medium. In addition, the flow resistance should besufficiently low as to result in only a low pressure drop through theporous material. The magnetic field volume should preferably beminimized.

Stacks of heat exchanger beds or monoliths can be thermally insulatedfrom one another by appropriate intermediate layers, for example bycarbon sieves. This prevents heat losses as a result of conduction ofheat within the material. By virtue of appropriate design, theintermediate layers may also serve for homogeneous distribution of theheat exchanger medium.

The heat exchanger beds obtained in accordance with the invention arepreferably used in refrigerators, air conditioning units, heat pumps orheat exchangers, or in power generation by direct conversion of heat.The materials should exhibit a large magnetocaloric effect within atemperature range between −100° C. and +150° C.

The heat transfer rate limits the cycle speed and hence has a greatinfluence on the power density.

In power generation, a coil of an electrically conductive material isarranged around the thermomagnetic material. In this coil, a current isinduced through alteration of the magnetic field or of themagnetization, and can be used to perform electrical work. Preference isgiven to selecting the coil geometry and the geometry of thethermomagnetic material so as to result in a maximum energy yield withminimum pressure drop. The coil winding density (turns/length), the coillength, the charge resistance and the temperature change of thethermomagnetic material are important influencing parameters for theenergy yield.

The thermomagnetic material is present in an external magnetic field.This magnetic field can be generated by permanent magnets orelectromagnets. Electromagnets may be conventional electromagnets orsuperconductive magnets.

The thermomagnetic generator is preferably designed such that thethermal energy from geothermal sources or from the waste heat ofindustrial processes or from solar energy or solar collectors can beconverted, for example, in photovoltaics. Specifically in regions withgeothermal activity, the inventive thermomagnetic generator allowssimple power generation exploiting geothermal heat. In industrialprocesses, process heat or waste heat frequently arises, which istypically discharged to the environment and is not utilized further.Wastewaters frequently also have a higher temperature on exit than onentry. The same applies to cooling water. The thermomagnetic generatorthus allows the recovery of electrical energy from waste heat which isotherwise lost. By virtue of the fact that the thermomagnetic generatorcan be operated in the region of room temperature, it is possible toutilize this waste heat and to convert it to electrical energy. Theenergy conversion is effected preferably at temperatures in the rangefrom 20 to 150° C., more preferably at temperatures in the range from 40to 120° C.

In (concentrated) photovoltaic systems, high temperatures are frequentlyattained, such that it is necessary to cool. This heat to be removed canbe converted to power in accordance with the invention.

For power generation, the thermomagnetic material is contactedalternately with a warm reservoir and a cool reservoir and hencesubjected to a warming and cooling cycle. The cycle time is selectedaccording to the particular technical prerequisites.

The examples which follow describe the production of thermomagneticmaterials suitable for the inventive application and the design ofmonoliths and catalyst beds.

EXAMPLES Example 1

Evacuated quartz ampoules which comprised pressed samples of MnFePGewere kept at 1100° C. for 10 hours in order to sinter the powder. Thissintering was followed by heat treatment at 650° C. for 60 hours inorder to bring about homogenization. Instead of slow cooling in the ovento room temperature, the samples were, however, immediately quenched inwater at room temperature. The quenching in water caused a certaindegree of oxidation at the sample surfaces. The outer oxidized shell wasremoved by etching with dilute acid. The XRD patterns showed that allsamples crystallized in a structure of the Fe₂P type.

The following compositions were obtained:

Mn_(1.1)Fe_(0.9)P_(0.81)Ge_(0.19); Mn_(1.1)Fe_(0.9)P_(0.78)Ge_(0.22),Mn_(1.1)Fe_(0.9)P_(0.75)Ge_(0.25) and Mn_(1.2)Fe_(0.8)P_(0.81)Ge_(0.19).The values observed for the thermal hysteresis are 7 K, 5 K, 2 K and 3 Kfor these samples in the given sequence. Compared to a slowly cooledsample, which has a thermal hysteresis of more than 10 K, the thermalhysteresis has been greatly reduced.

The thermal hysteresis was determined in a magnetic field of 0.5 tesla.

The Curie temperature can be adjusted by varying the Mn/Fe ratio and theGe concentration, as can the value of the thermal hysteresis.

The change in the magnetic entropy, calculated from the direct currentmagnetization using the Maxwell relationship, for a maximum field changeof from 0 to 2 tesla, is 14 J/kgK, 20 J/kgK and 12.7 J/kgK respectivelyfor the first three samples.

The Curie temperature and the thermal hysteresis decrease withincreasing Mn/Fe ratio. As a result, the MnFePGe compounds exhibitrelatively large MCE values in a low field. The thermal hysteresis ofthese materials is very low.

Example 2 Melt-Spinning of MnFeP(GeSb)

The polycrystalline MnFeP(Ge, Sb) alloys were first produced in a ballmill with high energy input and by solid phase reaction methods, asdescribed in WO 2004/068512 and J. Appl. Phys. 99, 08 Q107 (2006). Thematerial pieces were then introduced into a quartz tube with a nozzle.The chamber was evacuated to a vacuum of 10⁻² mbar and then filled withhigh-purity argon gas. The samples were melted by means of a highfrequency and sprayed through the nozzle owing to a pressure differenceto a chamber containing a rotating copper drum. The surface speed of thecopper wheel was adjustable, and cooling rates of about 10⁵ K/s wereachieved. Subsequently, the spun ribbons were heat treated at 900° C.for one hour.

X-ray diffractometry reveals that all samples crystallize in thehexagonal Fe₂P structure pattern. In contrast to samples not produced bythe melt-spinning method, no smaller contaminant phase of MnO wasobserved.

The resulting values for the Curie temperature, the hysteresis and theentropy were determined for different peripheral speeds in themelt-spinning. The results are listed in Tables 1 and 2 which follow. Ineach case, low hysteresis temperatures were determined.

TABLE 1 V (m/s) T_(c) (K) ΔT_(hys) (K) −ΔS (J/kgK) RibbonsMn_(1.2)Fe_(0.8)P_(0.73)Ge_(0.25)Sb_(0.02) 30 269 4 12.1Mn_(1.2)Fe_(0.8)P_(0.70)Ge_(0.20)Sb_(0.10) 30 304 4.5 19.0 45 314 3 11.0MnFeP_(0.70)Ge_(0.20)Sb_(0.10) 20 306 8 17.2 30 340 3 9.5MnFeP_(0.75)Ge_(0.25) 20 316 9 13.5 40 302 8 —Mn_(1.1)Fe_(0.9)P_(0.78)Ge_(0.22) 20 302 5 — 40 299 7 —Mn_(1.1)Fe_(0.9)P_(0.75)Ge_(0.25) 30 283 9 11.2Mn_(1.2)Fe_(0.8)P_(0.75)Ge_(0.25) 30 240 8 14.2Mn_(1.1)Fe_(0.9)P_(0.73)Ge_(0.27) 30 262 5 10.1 BulkMnFeP_(0.75)Ge_(0.25) 327 3 11.0 Mn_(1.1)Fe_(0.9)P_(0.81)Ge_(0.19) 260 714.0 Mn_(1.1)Fe_(0.9)P_(0.78)Ge_(0.22) 296 5 20.0Mn_(1.1)Fe_(0.9)P_(0.75)Ge_(0.25) 330 2 13.0Mn_(1.2)Fe_(0.8)P_(0.81)Ge_(0.19) 220 3 7.7Mn_(1.2)Fe_(0.8)P_(0.75)Ge_(0.25) 305 3 —Mn_(1.2)Fe_(0.8)P_(0.73)Ge_(0.27) 313 5 —Mn_(1.3)Fe_(0.7)P_(0.78)Ge_(0.22) 203 3 5.1Mn_(1.3)Fe_(0.7)P_(0.75)Ge_(0.25) 264 1 —

TABLE 2 T_(c) (K) ΔT_(hys) (K) −ΔS (J/kgK) Bulk MnFeP_(0.75)Ge_(0.25)327 3 11.0 Mn_(1.16)Fe_(0.84)P_(0.75)Ge_(0.25) 330 5 22.5Mn_(1.18)Fe_(0.82)P_(0.75)Ge_(0.25) 310 3 16.1Mn_(1.20)Fe_(0.80)P_(0.75)Ge_(0.25) 302 1 12.0Mn_(1.22)Fe_(0.78)P_(0.75)Ge_(0.25) 276 4 11.7Mn_(1.26)Fe_(0.74)P_(0.75)Ge_(0.25) 270 1 8.5Mn_(1.1)Fe_(0.9)P_(0.81)Ge_(0.19) 260 6 13.8Mn_(1.1)Fe_(0.9)P_(0.78)Ge_(0.22) 296 4 20.0Mn_(1.1)Fe_(0.9)P_(0.77)Ge_(0.23) 312 2 14.6Mn_(1.1)Fe_(0.9)P_(0.75)Ge_(0.25) 329 2 13.0 RibbonsMn_(1.20)Fe_(0.80)P_(0.75)Ge_(0.25) 288 1 20.3Mn_(1.22)Fe_(0.78)P_(0.75)Ge_(0.25) 274 2 15.3Mn_(1.24)Fe_(0.76)P_(0.75)Ge_(0.25) 254 2 16.4Mn_(1.26)Fe_(0.74)P_(0.75)Ge_(0.25) 250 4 14.4Mn_(1.30)Fe_(0.70)P_(0.75)Ge_(0.25) 230 0 9.8

Example 3

A simple magnetocaloric regenerator formed from a packed or structuredbed of a cascade of magnetocaloric materials a magnet arrangement and aheat transfer fluid was examined with the following results:

1. For given operating conditions:

Form of the magnetocaloric material Power (W) Pressure drop (Pa)Spheres, d = 0.3 mm 160.5  1.8 × 10⁴ Spheres, d = 0.05 mm 162.9  5.6 ×10⁵ Spheres, d = 0.03 mm 163.0 1.52 × 10⁶ Monolith, channels with squarecross 154.6 1.71 × 10³ section, side length 0.3 mm Cylinder, d = h = 5mm Heat transfer too slow to build up a temperature gradient Cylinder, d= h = 0.5 mm 154.3 9.11 × 10³

It is evident that spheres of diameter 0.3 mm and the monolith give riseto good heat transfer performances, while only low pressure dropsoccurred (especially in the monolith).

2. For operation at different frequencies (all other operatingconditions remaining the same):

The table which follows lists the net power (which is the cooling powerminus the power required for the pumping of the heat transfer fluid) atdifferent operating frequencies:

Power at different Form of the magnetocaloric operating frequencies (W)material 1 Hz 5 Hz 10 Hz Monoliths, square channels, 153.2 645.21024.6   side length 0.1 mm Monoliths, square channels, 108.6   0* 0*side length 0.3 mm Spheres, d = 0.05 mm 96.1 522.2 986.1   Spheres, d =0.3 mm 90.1 259.1 0* N.B.: 0* means that the power required for thepumping of the fluid is greater than the cooling power obtained.

1. A packed heat exchanger bed composed of thermomagnetic materialparticles which have a mean diameter in the range from 50 μm to 400 μmand give rise to a porosity in the packed bed of from 30 to 45%.
 2. Theheat exchanger bed according to claim 1, wherein the material particleshave a mean diameter of from 50 μm to 200 μm.
 3. The heat exchanger bedaccording to claim 1, wherein the material particles are in sphericalform, pellet form, sheet form or cylinder form.
 4. The heat exchangerbed according to claim 1, wherein the porosity of the packed bed is 36to 45%.
 5. A heat exchanger bed composed of a thermomagnetic materialmonolith which has continuous channels with a cross-sectional area ofthe individual channels of from 0.001 to 0.2 mm² and a wall thickness of50 to 300 μm, a porosity of from 10 to 60% and a ratio of surface tovolume of from 3,000 to 50,000 m²/m³, or which has a plurality ofparallel sheets with a sheet thickness of 0.1 to 2 mm and a sheetseparation of 0.05 to 1 mm.
 6. The heat exchanger bed according to claim5, wherein the porosity is 20 to 30%.
 7. The heat exchanger bedaccording to claim 5, wherein the cross-sectional area of the individualchannels is from 0.01 to 0.03 mm² and the wall thickness is from 50 to150 μm.
 8. The heat exchanger bed according to claim 1, wherein thethermomagnetic material is at least one selected from the groupconsisting of (1)-(8): (1) at least one compound of the formula (I)(A_(y)B_(1-y))_(2+δ)C_(w)D_(x)E_(z)   (I),  wherein  A is Mn or Co,  Bis Fe, Cr or Ni,  C, D and E at least two of C, D and E are different,have a non-vanishing concentration and each independently is selectedfrom the group consisting of P, B, Se, Ge, Ga, Si, Sn, N, As and Sb,where at least one of C, D and E is Ge or Si,  δ is a number of from−0.1 to 0.1,  w, x, y, z each independently is from 0 to 1, whereinw+x+z=1; (2) at least one La- and Fe-based compound of the formulae (II)and/or (III) and/or (IV)La(Fe_(x)Al_(1-x))₁₃H_(y) or La(Fe_(x)Si_(1-x))₁₃H_(y)   (II)  wherein x is a number of from 0.7 to 0.95,  y is a number of from 0 to 3;La(Fe_(x)Al_(y)Co_(z))₁₃ or La(Fe_(x)Si_(y)Co_(z))₁₃   (III),  wherein x is a number of from 0.7 to 0.95,  y is a number of from 0.05 to 1−x, z is a number of from 0.005 to 0.5;LaMn_(x)Fe_(2-x)Ge   (IV),  wherein  x is a number of from 1.7 to 1.95,and (3) at least one Heusler alloy of the MnTP type, wherein T is atransition metal and P is a p-doping metal having an electron count peratom e/a of from 7 to 8.5, (4) at least one Gd- and Si-based compound ofthe formula (V)Gd₅(Si_(x)Ge_(1-x))₄   (V)  wherein x is a number of from 0.2 to 1, (5)at least one Fe₂P-based compound, (6) at least one manganite of theperovskite type, (7) at least one compound which comprises at least onerare earth element and has the formulae selected from the groupconsisting of (VI) and (VII)Tb₅(Si_(4-x)Ge_(x))   (VI),  wherein x=0, 1, 2, 3, or 4,XTiGe   (VII),  wherein X=Dy, Ho, or Tm, (8) at least one Mn- and Sb- orAs-based compound of the formulae (VIII) and (IX)Mn_(2-x)Z_(x)Sb   (VIII)Mn₂Z_(x)Sb_(1-x)   (IX)  wherein  Z is Cr, Cu, Zn, Co, V, As, or Ge,  xis from 0.01 to 0.5,  wherein Sb may be replaced by As when Z is not As.9. The heat exchanger bed according to claim 8, wherein thethermomagnetic material is at least one quaternary compound of theformula (I) which, in addition to Mn, Fe, P and optionally Sb, furthercomprises Ge; Si; As; Ge and Si; Ge and As; Si and As; or Ge, Si, andAs.
 10. A process for producing heat exchanger beds according to claim1, comprising subjecting a powder of the thermomagnetic material toshaping to form the thermomagnetic material particles and subsequentlypacking the material particles to form the heat exchanger bed.
 11. Aprocess for producing heat exchanger beds according to claim 5 byextrusion, injection molding or molding of the thermomagnetic materialto form the monolith.
 12. A refrigerator, an air conditioning unit, aheat pump, or a thermomagnetic generator comprising the packed heatexchanger bed according to claim 1.